Conventionally, stacked piezoelectric devices are used as drive source of fuel injectors. The stacked piezoelectric device is made up of, for example, a ceramic laminate formed by stacking inner electrodes and piezoelectric ceramics alternately and a pair of outer electrodes connected to the inner electrode alternately.
The stacked piezoelectric device is used in severe environmental conditions over a long duration, especially when employed in fuel injectors. Therefore, in order to improve the electric insulation of the side surfaces, a ceramic laminate having inner electrode-unformed areas where a portion of an end of an inner electrode layer is recessed inwardly is adapted widely.
However, the formation of the inner electrode-unformed areas in order to improve the insulation may cause portions which are susceptible and insusceptible to deformation to appear in the ceramic laminate upon application of voltage thereto, resulting in concentration of stress at interfaces therebetween and cracks in the device.
In order to avoid the cracks arising from the concentration of stress, stacked piezoelectric devices are being developed which have grooves (stress absorbing portions) formed at a given interval away from each other in a laminating direction in the side surface of the ceramic laminate (see patent document 1).
However, even when the stress absorbing portions are formed, the application of the voltage to the stress absorbing portions also may result in cracks extending from the top end of the stress absorbing portions. In order to avoid this, it is necessary to increase the depth of the stress absorbing portion in a direction perpendicular to the laminating direction more than the distance of the inner electrode-unformed areas. Such a structure, however, causes great electric discharge to occur at the stress absorbing portions (grooves) upon application of great voltage thereto, so that they may be short-circuited. This gives rise to the problem of insufficient electric insulation, which results in a decrease in service life of the stacked piezoelectric devices.
Stacked piezoelectric devices are being developed in which the inner electrodes interleaving the stress absorbing portion therebetween are made to have the same polarity in order to avoid the formation of cracks (see patent document 2)
In such conventional stacked piezoelectric devices, it is possible to make the inner electrodes interleaving the stress absorbing portion therebetween to have the same polarity to make the piezoelectric ceramic layer interleaved between them as voltage inactive layers, thereby concentrating the stress at the voltage inactive layers when the stacked piezoelectric device expands. This causes cracks to occur in the stress absorbing portions selectively or preferentially, thereby avoiding the crack in voltage active layers of the laminate to improve the durability.
However, in fact, even when no cracks occur in the stress absorbing portions, it is still difficult to ensure sufficient electric insulation, which gives rise to the problem of a drop in electric insulation, thus resulting in formation of cracks. Additionally, electric field is not applied to the ceramic layers including the stress absorbing portions interleaved between the inner electrodes having the same polarity, so that they are hardly deformed. This results in the problem of a decreased amount of stroke of the stacked piezoelectric devices.